1. Field of the Invention
The present invention relates to a thin zoom optical system, an imaging lens device incorporated with the zoom optical system, and a digital apparatus loaded with the imaging lens device.
2. Description of the Related Art
In recent years, with an explosive spread of digital apparatuses such as a digital still camera, a digital video camera, a mobile phone with a built-in camera (hereinafter, called as “camera phone”), and a personal digital assistant (PDA), development of a high-resolution or sophisticated image sensor to be loaded in these digital apparatuses has been rapidly progressed. In view of this, high optical performance is demanded for a zoom optical system for guiding an optical image of a subject to an image sensor in order to sufficiently utilize the performance of the high-resolution image sensor.
Also, in digital apparatuses for general use, there is a demand for optical zooming capable of zooming an image, particularly, with less image degradation. In addition to this demand, miniaturization is required to enhance portability. There is proposed reducing the thickness of the zoom optical system as one measure for miniaturization of the digital apparatus. Conventionally, a collapsible mechanism has been adopted in the zoom optical system as one measure for miniaturization of the zoom optical system, for instance.
In the zoom optical system adopting the collapsible mechanism, the construction of a lens barrel is complicated, which may give rise to cost increase. Further, in a mechanism constructed such that a lens unit pops out in response to turning on of the power of the digital apparatus, it takes a certain time to finalize a shooting preparatory operation. Accordingly, a user may fail to release the shutter at a right moment to capture a scene.
There is known a technique of providing a reflecting surface on an optical path of a zoom optical system, as another measure for reducing the thickness of the zoom optical system. Various arrangements have been proposed in the zoom optical system. For instance, Japanese Unexamined Patent Publication No. 2004-70235 (called as “D1”) discloses a zoom optical system, wherein an optical axis is bent by 90 degrees by fixedly arranging a triangular prism in a lens group closest to an object or a subject, and an incident surface of the triangular prism for passing an incident ray is shaped into an aspherical concave surface. Japanese Unexamined Patent Publication No. 2004-170707 or counterpart U.S. Patent Application Publication No. 2004/0095503A1 (called as “D2”) discloses a technique of miniaturizing a zoom optical system by providing two reflecting surfaces for bending an optical axis by 90 degrees, wherein the bending direction is “twisted” in the space. Japanese PCT Publication (tokuhyo) 2000-515255 or counterpart U.S. Pat. No. 6,850,279B1 (called as “D3”) discloses a technique of miniaturizing an optical system by providing two reflecting elements, namely, mirrors for bending an optical axis by 90 degrees in a fixed focal length optical system. Japanese Unexamined Patent Publication No. 2004-247887 (called as “D4”) discloses a technique of miniaturizing an optical system by providing two reflecting elements such as a triangular prism or a mirror for bending an optical axis by 90 degrees.
The zoom optical system recited in D1 uses the triangular prism whose incident surface is an aspherical concave surface in an attempt to realize a thin, compact zoom optical system with its thickness thereof being reduced. In the zoom optical system recited in D1, since the optical axis is bent once, the thickness of the camera incorporated with the zoom optical system is determined by the size of the image sensor. Generally, parts such as a wiring, a circuit, and a packaging unit are arranged in the periphery of a light receiving plane of an image sensor, and the areas of these parts are considerably large as compared with the area of the light receiving plane. Therefore, the arrangement of D1 needs further improvement for miniaturization.
The zoom optical system recited in D2 has two reflecting surfaces for bending the optical axis by 90 degrees. However, the bending direction is “twisted” in the space. Accordingly, as in the case of D1, the thickness of the camera loaded with the optical system is determined by the size of the image sensor, and the arrangement of D2 needs further improvement for miniaturization.
In the optical system recited in D3, the thickness of the camera is determined by the thickness of the optical system, because the two reflecting surfaces are provided to bend the optical axis by 90 degrees, and the object-side surface of the optical system is made parallel with the imaging plane of the image sensor. Since the optical system recited in D3 is a fixed focal length optical system, a zoom optical system is not provided. Further, the optical axis is bent by using a reflecting mirror, the required optical path is long as compared with the case of using a prism. As a result, the thickness of the optical system at a portion where the optical axis is bent is increased.
Although a prism is used for bending the optical axis in the optical system recited in D4, the prism is a triangular prism of a simple construction. Further, since a lens element is provided on the object side outside of the prism, the arrangement of D4 needs further improvement for miniaturization. Furthermore, the lens element is used merely as a photographing window, and D4 does not disclose an idea of effectively utilizing an effect by the provision of the lens element. As mentioned above, the optical systems disclosed in D1 through D4 have failed to provide a high-performance zoom optical system, and there is room for further miniaturization in these optical systems.